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Highlights • Cokes generated at slow heating rates are imporous with smooth surface. • The radical concentration of cokes reaches highest at 600 °C then decreases rapidly. • Interactions among bio-oil components bring the O-functional groups into the coke. • The cokes are more condensed with bigger aromatic rings at high temperatures. Abstract Coke can be formed once the bio-oil was heated, even at very low temperatures, causing almost always serious problems in the upgrading and direct utilization of bio-oil. To minimize the negative impacts from coke formation, the key point is to fully understand the formation and evolution of coke during the thermal treatment of bio-oil. Thus, in this study, the cokes formed from the pyrolysis of bio-oil at different temperatures (300–800 °C) and heating rates were characterized by using a range of advanced analytical instruments. The evolution of cokes (e.g. morphology, elemental composition, chemical structure and concentration of radicals) with increasing temperature and heating rate was traced. The results show that the cokes generated at slow heating rates are imporous with smooth surface but porous at high temperatures and fast heating rates. The radical concentration of the cokes reaches the highest level at 600 °C, then decreases rapidly with further increasing temperature to form more free radicals, which promote the condensation reaction of aromatic systems to form larger ring structures of the coke at high temperatures, with lower H/C and O/C ratios. The O-containing functional groups could be brought into the coke via the interactions between light and heavy components of bio-oil. [ABSTRACT FROM AUTHOR]

A study was carried out in both inert and CO 2 atmosphere to investigate the interaction mechanism of K 2 CO 3 with graphite and coal char during the catalytic gasification of coal char. The formation of K 2 CO 3 catalytic precursors was identified by using series of different techniques. The results obtained in an inert atmosphere show that the interaction between K 2 CO 3 and carbon readily took place at low temperature. For the K 2 CO 3 loaded graphite, the intermediate of C n K clusters formed by the interactions of K 2 CO 3 with carbon prevented the evaporation of K at low temperature. At temperatures above 800 °C, however, the C n K clusters were unstable, and decomposed to release K through vaporization. On the contrary, for the K 2 CO 3 loaded demineralized coal char, due to the presence of oxygen groups, K was tightly bonded to carbon during heating, forming a complex precursor containing O and K with high thermal stability. The reaction pathway of carbon-K 2 CO 3 interactions in an inert atmosphere are proposed for both char and graphite. The results of DRIFTs experiment suggest that CO 2 is chemisorbed by the K- catalytic precursors during CO 2 gasification to form potassium phenolate species and a covalently bound carbonate group (dimethyl carbonate). It is believed that these species act as active intermediates in the gasification process. [ABSTRACT FROM AUTHOR]

• Char activity in volatile-char interactions varies with biomass components. • Xylan char rich in C O group is the most active for (benzyloxy)benzene conversion. • Char C(sp2) O group promotes the breaking of C(sp3) O in (benzyloxy)benzene. • Char C(sp3) O group promotes the breaking of C(sp2) O in (benzyloxy)benzene. • Lignin char is highly selective to monomers due to the abundant C O group. Char activity is a critical factor for the interactions between char and volatile, which are recognized to determine the final product distribution from biomass pyrolysis. To clarify the effect of biomass composition on char activity, the interactions between (benzyloxy)benzene as a volatile model compound and char samples obtained from cellulose, xylan, and lignin were studied. The (benzyloxy)benzene conversion increased from 0.64 % for no char to 6.31 %, 6.98 % and 53.5 % for lignin char, cellulose char, and xylan char, respectively, which were mainly derived from the contribution of surface C O groups and surface defects in the carbon structure. Especially, surface C(sp2) O groups and C(sp3) O groups on char respectively favored the breakage of C(sp3) O and C(sp2) O bonds in (benzyloxy)benzene. Despite the low activity, lignin char showed high selectivity to monomeric products due to the high content of surface C O groups, which were considered to induce the production of hydrogen radicals to stabilize the resulting intermediates. The findings of this study prove that the volatile-char interaction could be adjusted facilely by changing the hemicellulose content during feedstock pretreatments. [ABSTRACT FROM AUTHOR]